1. Technical Field
The present disclosure relates to an array substrate, and more particularly, to an array substrate including an oxide thin film transistor and a method of fabricating the same.
2. Discussion of the Related Art
Recently, as the information society progresses, display devices processing and displaying a large amount of information have rapidly advanced and various flat panel displays (FPDs) have been developed. Specifically, the FPDs such as a liquid crystal display (LCD) device, a plasma display panel (PDP) device, an organic light emitting diode (OLED) display device and a field emission display (FED) device having a superior performance with a thin profile, a light weight, and a low power consumption have substituted for a cathode ray tube (CRT).
Among various FPDs, the LCD device having a high contrast ratio, an advantage in displaying a moving image and a low power consumption has been used in various fields such as a notebook, a monitor and a television. The LCD device uses an optical anisotropy and a polarization property of a liquid crystal molecule.
In addition, the OLED display device has superior properties such as a high brightness and an ability to be driven by a low voltage. Since the OLED display device is an emissive type, the OLED display device has a high contrast ratio and a thin profile. The OLED display device has an advantage in displaying a moving image due to a short response time of several micro seconds (μsec). The OLED display device has no limitation on a viewing angle and is stable even in a low temperature.
The LCD device and the OLED display device include an array substrate having a thin film transistor (TFT) for turning on and off a pixel region. In general, the TFT is formed by using a semiconductor material such as amorphous silicon.
Recently, as size and resolution of the FPD increase, a TFT having a faster processing speed, more stable operation and durability is required. However, since the TFT using amorphous silicon has a charge carrier mobility smaller than about 1 cm2/Vsec, the TFT using amorphous silicon has a limit for application to the FPD having a great size and a high resolution.
Accordingly, an oxide TFT where an active layer is formed of an oxide semiconductor material having a high mobility and a uniform current property has been widely researched.
FIG. 1 is a plan view showing a gate line of an array substrate according to the related art, and FIG. 2 is a cross-sectional view showing an oxide thin film transistor of an array substrate according to the related art.
In FIGS. 1 and 2, a gate line 24 is formed along a first direction on a substrate 10 and includes a gate electrode 23 corresponding to a gate electrode region L1. A gate insulating layer 30, an active layer 40, an etch stopping layer 45 and source and drain electrodes 52 and 54 are sequentially formed on the gate electrode 23 to constitute an oxide thin film transistor (TFT). In addition, a passivation layer 60 and a pixel electrode 70 are sequentially formed on the oxide TFT. The passivation layer 60 has a drain contact hole 62 exposing the drain electrode 54 and the pixel electrode 70 is connected to the drain electrode 54 through the drain contact hole 62.
The gate line 24 including the gate electrode 23 is formed of copper (Cu) having a relatively low resistance, a relatively high electrical conductivity and a relatively low dielectric constant. Since a plurality of heat treatment processes are performed for fabricating the oxide TFT, grains of copper may grow on a top surface of the gate electrode 23 due to the heat.
FIG. 3A is a cross-sectional view showing grain growth on a top surface of a gate electrode of an array substrate according to the related art, and FIG. 3B is a scanning electron microscope image showing a portion AA of FIG. 3A.
In FIGS. 3A and 3B, a grain grows on a top surface in a central portion of the gate electrode 23 due to the heat. The grain may protrude from the top surface to form a protrusion 23a and a cavity in the gate electrode 23 and the gate line 24. In addition, the protrusion 23a may cause a protrusion of the gate insulating layer 30, the active layer 40 and the passivation layer 60 sequentially on the gate electrode 23 such that the gate insulating layer 30, the active layer 40 and the passivation layer 60 do not have a flat top surface. Since a property of the oxide TFT is deteriorated due to the protrusion, reliability of an array substrate including the oxide TFT is reduced and defect rate of a display device using the array substrate increases.
Further, grain growth and cavity formation may occur in a metal line having a metallic material different from copper (Cu) and an array substrate including the metal line may be deteriorated.